1. Technical Field
The present invention relates to a method for driving an ultrasonic motor.
2. Related Art
Ultrasonic motors are one type of the devices that convert electric energy to mechanical energy. One example of such ultrasonic motors may be formed from a rotor with an operation shaft and a piezoelectric actuator in which a tip of the piezoelectric actuator is pressed against a side surface of the rotor. According to this type of ultrasonic motor, the tip of the piezoelectric actuator is vibrated in an elliptical motion in a plane perpendicular to the axis of the rotor, whereby the side surface of the rotor is pushed by the tip of the piezoelectric actuator thereby generating a moment in the rotor, and thus generating a rotational movement of the rotor with its axis as a rotation center. This type of ultrasonic motor is described, for example, in JP-A-2004-166324.
Generally, in ultrasonic motors, the closer the frequency of vibration of the piezoelectric actuator to the resonance frequency of the piezoelectric actuator, the faster the rotation speed of the motor becomes. In this respect, various contrivances have been made in the configuration and arrangement of components in the motor, the manner of vibration of the tip of the piezoelectric actuator, etc.
However, at vibrations near the resonance frequency of the piezoelectric actuator, the piezoelectric actuator may not start vibrating unless the piezoelectric actuator is vibrated at a predetermined frequency or higher, or may start but unexpectedly stop when its vibration lowers below the resonance frequency even slightly due to various reasons, for example, because the tip of the piezoelectric actuator is pressed against the rotor. Also, the resonance frequency is influenced by the surrounding conditions, changes in the temperature of the motor, and the like. Because of the phenomenon described above, stable driving of an ultrasonic motor presents one of the objects to be overcome.